Great attention is now paid to optical recording media capable of high density recording and erasing the once recorded information for rewriting. Among such rewritable optical recording media, phase change recording media are designed such that recording is performed by irradiating a laser beam to a recording layer to change its crystalline state and reading is performed by detecting the change of reflectance of the recording layer associated with that state change. The phase change optical recording media are of greater interest because the drive unit may have a simple optical system as compared with that used for magneto-optical recording media.
For the phase change recording layer, calcogenide materials such as Ge—Sb—Te are often used because of a greater difference in reflectance between the crystalline and amorphous states and a relatively high stability in the amorphous state.
When information is recorded in a phase change optical recording medium, the recording layer is irradiated with a laser beam having a high power (recording power) sufficient to heat the recording layer at or above its melting point. In the region where the recording power is applied, the recording layer is melted and then rapidly cooled, forming a recorded mark in the amorphous state. The recorded mark is erased by irradiating the recording layer with a laser beam having a relatively low power (erasing power) sufficient to heat the recording layer above its crystallization temperature, but below its melting point. The recorded mark to which the erasing power is applied is heated above the crystallization temperature and then slowly cooled, resuming the crystalline state. Therefore, the phase change optical recording medium allows for overwriting simply by modulating the intensity of a single laser beam.
In order to increase the recording density and transfer rate of a recording medium, attempts have been made to reduce the wavelength of recording/reading light, to increase the numerical aperture of an objective lens in a recording/reading optical system, and to increase the linear velocity of the medium. The diameter of a spot that is defined on the surface of the recording layer by a recording laser beam is represented by λ/NA wherein λ is the wavelength of the laser beam and NA is the numerical aperture. The spot diameter λ/NA divided by the linear velocity V of the medium, i.e., (λ/NA)/V gives the time of irradiation of laser beam to the recording layer, that is, the time taken for passage across a beam spot. As the recording density and transfer rate increase, the irradiation time of laser beam to the recording layer becomes shorter and shorter. This makes it difficult to optimize overwriting conditions.
Problems arising from overwriting at an increased linear velocity are discussed below.
An increased linear velocity leads to a shortened irradiation time of recording light. It is then a common practice to increase the recording power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering. However, an increased linear velocity entails an increased cooling rate following irradiation of recording light. To form amorphous recorded marks, the recording layer once melted by irradiation of recording light must be cooled at a rate above a certain level corresponding to the crystallization rate. If the construction of the recording layer and the thermal design of the medium are the same, the cooling rate of the recording layer depends on the linear velocity. That is, the cooling rate becomes faster at higher linear velocities and becomes slower at lower linear velocities.
On the other hand, to erase the amorphous recorded mark (to recrystallize), an erasing beam must be irradiated such that the recording layer may be held for at least a predetermined time at a temperature between the crystallization temperature and the melting point. The attempt to increase the erasing power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering has a less likelihood to erase the recorded mark because the irradiation time is reduced as a result of the increased linear velocity.
Therefore, to increase the linear velocity for improving the data transfer rate, the recording layer must be formed of a composition having a relatively high crystallization speed such that recrystallization is completed within a relatively short time (as disclosed in JP-A 1-78444 and JP-A 10-326436), or the medium must have the structure (slow cooling structure) that prevents heat release from the recording layer. Also, as described in JP-A 7-262613 and JP-A 8-63784, it is believed that the medium is preferably provided with a slow cooling structure for preventing any drop of recording sensitivity by an increased linear velocity.